Radial piston engines are used in many areas of industry. For instance, they are used in the form of radial piston motors as a drive means for vehicles. If a vehicle is equipped with drive means in the form of radial piston motors comprising pistons in engagement with a cam surface it is necessary to interrupt the engagement between said pistons and the cam surface, and to fixedly locate the pistons in their lower dead point position in case that the vehicle is to be towed with a relatively high speed. There are different ways to provide for the fixation of the pistons in that dead point position.
It is, for example possible to subject the housing of the radial piston motor to a slight overpressure (for instance 1 bar). Provided that the inlet and outlet of the pistons are without pressure, the pistons will be moved inwardly due to the overpressure existing within the housing, and, as a consequence the pistons are completely separated from the cam curve which is provided on a cam disc. Thus, the shaft of the motor and the cylinder block mounted thereon can freely rotate (including the pistons which are located in that cylinder block), i.e. the desired free running function is achieved. However, to provide for the overpressure within the housing, an auxiliary pump is necessary to create that pressure and further, valve means are required, with the consequence that undesirable costs are encountered.
U.S. Pat. No. 4,469,012 discloses for a hydraulic motor a connection between its pistons by means of a spring. Spring forces will tend to pull the pistons into their lower dead point position. This design requires substantial structural elements as well as space for said springs which are located outside of the cylinder block. Moreover, relatively large spring biasing forces are necessary to maintain the pistons in their lowest position within the cylinder block. A similar design is also shown in the German laid open application 29 20 588. Further, German laid open application 24 17 348 discloses an annular spring. (Said laid open application 24 17 348 claims the following priority: Great Britain, Apr. 9, 1973, Serial Number 16895-73). U.K. patent application 2 128 265 A discloses an hydraulic motor or pump, wherein radial pistons are adapted to be returned to a retracted position inside their cylinders by springs. German laid open application 16 53 587 discloses another hydraulic motor. U.S. Pat. No. 4,716,816 discloses a hydraulic motor using a spring for pressing the piston into an inside position.